This project examines effects of drug use and HIV infection on measures of cerebral function in specific regions of the frontal lobe; the effect of drug use on neuropsychological function across stages of HIV infection; examines the role of cognitive impairment, progression of HIV infection and psychiatric disorder on severity and duration of drug usage; examines the longitudinal effects of drug use and its interaction with stage of HIV infection on the nature of neuropsychological dysfunction, the rate of cognitive decline, and the changes in cerebral metabolism.